Why is Arkansas so liberal?

Arkansas is located in the south, but its residents vote democratic. For example, 3 of its four house members are democrats. Both senators, as well as the current governor are members of the Democratic Party - though they are fairly conservative democrats. Of course, former Democratic President Bill Clinton was from Arkansas, and along with Tennessee, it is the only other state in the south with more registered democrats than republicans. Why is the Democratic Party so strong in Arkansas?